Hydrology – The interesting use case of Canals for Cal/Val activities

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Canals are very interesting for Cal/Val activities of satellite altimetry as they represent a simple case for evaluating the performances of altimetry over inland waters. Indeed, canals have the following valuable characteristics:

  • The canals have a controlled water level which results in a low temporal evolutionCanals are very calm which allow to avoid any surface roughness issues. The resulting radar echoes are very specular which is a simple case for satellite altimetry
  • No topography effect. It is thus easy to monitor with only one in-situ station.
  • Canals are easy sites to equip with in-situ stations and are most of the time well monitored with existing in-situ sensors.

During the St3TART project, a specific campaign was performed over the Canal du Midi, near the town of Trèbes (France). This site was chosen because the “Canal du Midi” is a little canal with a controlled water surface height, an ideal geometry (the canal is perpendicular to the Sentinel-3 track), and is therefore an ideal case to evaluate the best possible performances that can be achieved by Sentinel-3. The station is installed on a bridge crossing the “Canal du Midi”, just below a Sentinel-3 track.

Installation of the vortex.io Micro-Station on a bridge over the Canal du Midi

Once the computation of the reference height of the Micro-Station done, the station provides the precise height of the water level measured by the LiDAR. 

Time series of water surface height measured by the vorteX.io micro-station

The water surface height on “trèbes_1” is very stable, as expected.

These measurements can be used to compute a FRM (Fiducial Reference Measurement) and then compare it to Sentinel-3 data.

As the in-situ sensor is installed just under the satellite ground track, the computation of the FRM is very simple: there is no slope to be accounted for, no other in-situ data is required, and the measurement of the in-situ sensor can be performed at the exact same time as the satellite passes. The FRM simply corresponds to the measurement of the vorteX.io Micro-Station that has been configured to perform a measurement at the exact time of the Sentinel-3 pass.

The steps for the comparison are the following ones:

  1. Extraction of the water target of interest in the Topage database (French river mask database)
  2. Selection of the nearest data to the target centreline (coloured points) for each transect
  3. Computation of WSH for S3 and selection of in-situ measurement at the same time
  4. Computation of Root Mean Square Error (RMSE) between the two timeseries
Comparison between FRM and Sentinel-3 measurements on the Trèbes site

The results obtained on the comparison between the FRM on the Trèbes site and Sentinel-3A data is about 10 cm of RMSE for L2 PDGS products and 5.08 cm of RMSE with the new Sentinel-3 Thematic hydro products, which depicts the excellent performance of Sentinel-3A on this site, in accordance with the Mission Requirements.

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